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环节动物在进化上占有很重要的地位,是身体分节,具有真体腔的动物,大约有9000种。通常分为三个纲,即多毛纲、寡毛纲和蛭纲。多数生活在海水、淡水和土壤中。多毛纲动物是典型的海洋生物,一般生活在浅滩至四十米深的海底,其中有自由生活的沙蚕,管栖生活、夜间能发磷光的磷沙蚕,以及营寄生生活的吸口虫等。寡毛纲中也有水生和寄生的种类(如水丝蚓生活在淡水中),但主要是陆地
The annelids occupy an important position in evolution. They are divided into sections of the body and have true body cavity. There are about 9000 species. It is usually divided into three categories, namely, polychaeta, oligochaete, and rachis. Most live in seawater, fresh water and soil. The polychaetes are typical marine creatures. They usually live in the shallow depths of the shoal to the depth of 40 meters. They include the free-living sand cocoon, tube-dwelling life, phosphorous-enveloping phosphoryledon at night, and parasite-infested parasitic worms. . There are also aquatic and parasitic species in the oligochaete (eg, Hydrangea live in freshwater) but mainly land